clockevents: Simplify locking

Now that the notifier chain is gone there are no other users and it's
pointless to nest tick_device_lock inside of clockevents_lock because
there is no other use case.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20130425143436.162888472@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>